arcnet/com20020: add LEDS_CLASS dependency

The newly added led trigger support in the com20020-pci driver causes
build errors when CONFIG_LEDS_CLASS is disabled:

drivers/built-in.o: In function `com20020pci_probe':
(.text+0x185dc4): undefined reference to `devm_led_classdev_register'
(.text+0x185dd8): undefined reference to `devm_led_classdev_register'

This adds a Kconfig dependency to prevent the invalid configurations.
Other drivers appear to be split 50:50 between 'select' and 'depends on'
for this symbol, I picked 'depends on' as I could not find a common
policy and it generally causes fewer problems.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 8890624a ("arcnet: com20020-pci: add led trigger support")
Signed-off-by: David S. Miller <davem@davemloft.net>